1. Field of the Invention
The present invention relates to a relay unit capable of inquiring of a name resolution system, a terminal device, a communication system comprising the relay unit and the terminal device, and a communication control method. It should be noted in the present specification that a private IP address is referred to as “private address,” and a global IP address as “global address.”
2. Related Background Art
First of all, the communication system in which the relay unit (also called “router” hereinafter) is used and an address conversion method are described. A bridge between a global address space and a private address space is served via the router. Further, address conversion (in other words, conversion between a global address and a private address) is performed in the router by means of a NAT method, NAPT method, and IP masquerading method, thereby enabling communications.
It should be noted that a packet in the Internet cannot be used if either a transmission source address or a transmission destination address is not a global address, and a private address is limited in the scope of its use to the inside of a private network by a router.
A router holds, in a table, information for determining a router to which a packet is to be transferred next in accordance with a transmission destination of the packet. When a communication is made from a terminal device present in a private network to a terminal device present in a global network, the transmission source address of the packet sent to the router is converted into a global address that the router has, and a router for transferring the packet routing is determined in reference to the table, the router performing routing.
It should be noted that the address conversion methods described below are used in address conversion.
The NAT method is for converting a private address of a transmission source into a global address that the router has, and sending the converted address to a global network. Also, the information in which the private address is related to the global address of the router is held in the table, the global address of a transmission destination header is converted with a private address with respect to a packet which is sent from a terminal device of the global network to a terminal device of a private network, and the converted address is sent to the terminal device. In this method, the router has to hold the same number of global addresses as the total number of terminals which are connected to the global network simultaneously.
The NAPT method has a function for automatically converting a port number in addition to the function of the NAT method. With the port number being used for conversion, the use is allowed in a single global address of the router, even if a plurality of terminals exist in a private network. Furthermore, a table in which the terminals in the private network are related to the port numbers is held. However, the use may not be allowed in an application using a specific port number.
The IP masquerading method is a term used in Linux and has the same function as the NAPT method.
It should be noted that these systems may not be distinguished from one another and can also be broadly called “NAT.”
Incidentally, there is MPLS (Multi-Protocol Label Switching) as a communication system where labels are used (See Japanese Patent Application Laid-Open No. 2005-12352). This system is described simply hereinafter.
While a general router uses a transmission destination address of a packet in routing, the MPLS uses a label instead of an address. When a packet is sent to each of routers (LER: Label Edge Router) located at end points in a network which is configured with routers (LSR: Label Switching Router) having the MPLS function, a label is added to the packet, and this packet is transferred to the next LSR. Further, the LSR which received the packet performs routing based on the label and transfers the packet. When transferring the packet to a network outside the network configured with LSRs, LERs remove the packet and then transfer it.